This disclosure relates to systems and methods for production of high purity argon. More specifically, this disclosure relates to a system and a method for purifying crude argon, previously recovered from air using a cryogenic rectification column, via pressure swing adsorption at cold temperatures.
The production of argon from a cryogenic air separation unit (ASU) plant is known. The same ASU plant may also produce oxygen and/or nitrogen. Conventionally, the ASU plant will have high and low pressure distillation columns and a crude argon column, for example, as described in U.S. Pat. No. 5,313,800 to Howard et al. In some cases the crude argon column may be incorporated with the low pressure column in a divided wall configuration, for example, as described in U.S. Pat. No. 6,240,744 to Agrawal et al. For purposes of this disclosure, we disclose the production of argon, although it will be readily apparent to those of skill in the art, that oxygen and/or nitrogen enriched streams will be created that can be treated separately or returned to the ASU for further treatment.
Crude argon produced by cryogenic distillation, taken from the middle of an oxygen/nitrogen/argon separation column, can contain between 5 and 20% argon, less than 0.1% nitrogen, and balance oxygen. Crude argon taken from the top of an argon/oxygen separation column contains at least 50% by volume argon, less than 2% by volume nitrogen, and balance oxygen. More typically, this composition is at least 80% by volume argon, less than 0.5% by volume nitrogen, and balance oxygen. This level of purity is unsuitable for many end uses. Several methods have been employed to further purify the crude argon stream, including: the so-called deoxo or getter methods which require expensive metal catalysts/getters, on-site hydrogen, and potential hazards of uncontrolled exothermic reaction; cryogenic distillation alone which requires larger distillation columns that are cost prohibitive at smaller scale plants; or multiple adsorption processes each with their own drawbacks.
Several examples in the prior art (U.S. Pat. No. 2,810,454 to Jones et al., U.S. Pat. No. 3,928,004 to Allam et al., U.S. Pat. No. 3,996,028 to Golovko et al., U.S. Pat. No. 5,159,816 to Kovak et al.) utilize 4A zeolite (also known as NaA zeolite) in cryogenic adsorption processes to separate oxygen from argon. These examples teach adsorbing at feed temperatures below −100° C. to restrict argon from entering 4A pores and avoid significant argon co-adsorption. To regenerate, however, a temperature swing adsorption (TSA) cycle is taught, where heat must be applied to desorb oxygen from the adsorbent. At these regeneration temperatures, argon can enter 4A pores at a faster rate, and remain trapped in the pores if left in direct contact with the adsorbent during cool down, thereby reducing oxygen working capacity. This necessitates reduction of pressure below ambient by pulling vacuum on the adsorbent and then cooling by indirect means back to feed temperature. These thermal swings, coupled with vacuum pressure, increase the likelihood of contaminating the adsorbent with leaks to atmospheric gases.
The prior art also teaches methods to purify argon via pressure swing adsorption (PSA) or vacuum pressure swing adsorption (VPSA) at ambient temperatures using carbon molecular sieves (CMS) or other adsorbents (U.S. Pat. No. 5,730,003 to Nguyen, U.S. Pat. No. 7,501,009 to Graham et al., U.S. Pat. No. 6,500,235 to Zhong et al., Rege et al., U.S. Pat. No. 6,527,831 to Baksh et al.). These processes tend to provide lower recovery, less than 40%, unless power and capital intensive multi-train PSA processes are employed, comprising 2 or more sets of compressors and vessels (U.S. Pat. No. 6,500,235 to Zhong et al.). VPSA's also require vacuum during regeneration, increasing the chance for leaks to atmospheric contaminants. Verma et. al. describes gas uptake properties of CMS at 25° C., 0° C., and −84° C., showing enhancement of O2 selectivity over Ar as temperature is decreased, however no process information such as cycle schedule, number of beds, operating conditions etc. for improving process performance indicators in terms of argon recovery or argon productivity or argon purity is mentioned. Low argon recovery from the PSA can be mitigated by recycling waste gas from the PSA back to the cryogenic distillation plant. However when CMS is used, the process requires a means to ensure that carbon is not returned to the cryogenic distillation column. For example a filtration system can be used (U.S. Pat. No. 7,501,009 to Graham et al.). In that regard, it is desirable to reduce, if not eliminate, the amount of carbon used in the system.
Thus, it is desirable to develop systems and methods for purifying argon (Ar) gas that accomplish one or more of: improving argon recovery; requiring less equipment; requiring less energy; limiting waste streams; reducing recycle of waste streams by increasing yield; limiting the amount of carbon returned to the cryogenic distillation column; reducing the need for filtration of the recycle stream; reducing bed size; increasing productivity; and other improvements. Discussed herein are systems and methods of purifying a crude argon stream satisfying one or more of these desirable qualities.